L-lysine is not supported as a treatment or preventive for feline herpesvirus type 1 (FHV-1). A 2015 systematic review concluded that lysine supplementation is not effective for preventing or treating FHV-1 infection, and the ABCD feline herpesvirus guideline reports no evidence of efficacy and does not recommend lysine to prevent FHV re-excretion. No dose can be recommended on the basis of this evidence. If your cat has eye or breathing symptoms, the next step is a veterinary examination, not a supplement.
Why lysine was ever suggested for cats
FHV-1 is a common cause of upper respiratory and eye disease in cats. Once a cat is infected, the virus can remain latent in the body and reactivate later, often during stress such as a move to a new home or a course of corticosteroid medication. The idea behind lysine was that this amino acid might interfere with the virus, in part because lysine and arginine are related and the virus depends on arginine. That theory shaped a decade of supplement use and a number of small studies.
The theory did not hold up well. The 2015 systematic review by Bol and Bunnik, published in BMC Veterinary Research, explains that lysine does not antagonize arginine in cats in the way the proposed mechanism assumed. Cats cannot synthesize arginine themselves, so reducing arginine intake would not be a harmless way to test the idea. Arginine deficiency can cause hyperammonemia, a dangerous buildup of ammonia in the blood. The review’s point is that the mechanism is not a safe assumption to act on at home. (Bol and Bunnik, 2015, PubMed Central)
What the evidence shows
The 2015 systematic review
Bol and Bunnik examined seven studies of FHV-1 and lysine: five in cats and two in vitro (laboratory cell work). Their conclusion was that lysine supplementation is not effective for preventing or treating FHV-1 infection. The review also reports that some studies in cats found more frequent infection or more severe disease in lysine-treated groups. Those findings are a reason for caution, not just a reason to doubt a benefit. The review’s historical background notes that more than 90% of veterinarians in cat hospitals recommended lysine at the time it was written. That is a historical figure reported by the review, not a current estimate of veterinary practice.
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The ABCD feline herpesvirus guideline
The ABCD guideline on feline herpesvirus infection states that there is no evidence of efficacy for L-lysine and that it is not recommended to prevent FHV re-excretion. It also states that no side effects had been published in the literature it reviewed. Read that second statement carefully: it reports an absence of published side-effect reports for the studies it covers. It is not a finding that every lysine product, dose or duration is safe. (ABCD, Guideline for Feline Herpesvirus Infection)
The individual studies behind older advice
Several controlled studies, some of them small, explain why older recommendations or anecdotes sometimes sound more hopeful than the current consensus. Their results are mixed, and none of them set a clinical standard.
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| Study (year) | Cats and setting | Lysine protocol | Reported result |
|---|---|---|---|
| Maggs et al., 2003 (PubMed) | 14 cats, latently infected with FHV-1 | 400 mg once daily for 30 days | Fewer viral shedding episodes after a housing change, but not after corticosteroid-induced reactivation. Clinical signs did not differ significantly between groups. |
| Maggs et al., 2005 (PubMed) | Experimental conjunctivitis study; cat numbers not stated in the source summary | 500 mg every 12 hours | Less severe conjunctivitis. Viral isolation results did not differ between groups. |
| Drazenovich et al., 2009 (PubMed) | Shelter population; cat numbers not stated in the source summary | Dietary lysine at the amount studied | Did not successfully control infectious upper respiratory disease. |
| Rees and Lubinski, 2008 (PubMed) | Shelter population; cat numbers not stated in the source summary | Oral L-lysine | Did not prevent upper respiratory infection. |
The single improvement in the 2005 study, less severe conjunctivitis, was an isolated experimental outcome. It did not come with a difference in viral isolation, and the study does not show that the cats recovered faster or that lysine altered the course of infection in a way a pet owner could rely on.
Dosage: what the study numbers mean and what they do not
You will see 400 mg once daily and 500 mg every 12 hours quoted online. These are the protocols used in the studies above, under study conditions with defined outcomes. They are not dosage recommendations for a pet at home, and the sources do not establish a dose that is effective or appropriate for a given cat’s weight, age or health. Giving a cat a dose because it appeared in a study risks confusing a research protocol with clinical advice.
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- Targeted Eye Health: Primarily to aid against sticky eye discharge, this lysine supplement is specifically formulated to support healthy ocular function and overall eye wellness for cats and kittens of all ages
- Daily Immune Support: Vetoquinol Viralys is a maple-flavored L-Lysine gel for cats of all ages that supports immune strength and respiratory health for everyday pet wellness
- Respiratory Health Supplement: This gel supplement for kittens and cats offers a daily serving of lysine, helping to support normal respiratory health as part of a regular pet wellness routine
- Easy Feeding Oral Gel: The smooth gel supplement provides stress-free administration, allowing you to easily give it directly from the tube, mix it into your cat’s food, or apply it gently on your cat's paw or nose for easy feline care
- Confidence in Every Product: Made with a commitment to transparency, rigorous review, and dependable standards upheld across every product, we create happier and healthier moments for your pet
If a veterinarian has a specific reason to discuss a lysine product, the conversation should cover the cat’s diagnosis, what the product is expected to do, and how the cat will be monitored. The product’s label amount per serving and its ingredient list are facts you can check, but they do not indicate that a higher amount, a particular flavor or a specific format works better against FHV-1.
Side effects and safety
The ABCD guideline reports no published side effects in the literature it cites. That is a narrower statement than “lysine is safe.” The published studies were small, used different protocols, and did not evaluate long-term use across different products, doses or cats with other conditions. A cat with kidney disease, a liver condition or a complicated medication history is not covered by any of these studies.
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The safety concern that matters most is the arginine question. Because a cat cannot make its own arginine, any approach that lowers arginine in the body needs to be handled with great care, and the review’s warning about hyperammonemia is the clearest reason not to try to use lysine to block the virus through diet or supplements on your own.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.When to see a veterinarian
- Discharge from one or both eyes, squinting, or a third-eyelid film or swelling.
- Sneezing, nasal discharge, noisy breathing or reduced appetite that lasts more than a day or two.
- Any cat that is a kitten, older, pregnant, immune-compromised or on steroid medication, where symptoms can progress quickly.
- A cat that has been moved to a new home or boarded recently and shows respiratory or eye signs.
- Any plan to give lysine or another supplement alongside treatment, so the vet can check for interactions and monitor the cat.
Eye and respiratory disease in cats can have causes other than FHV-1, and some need prompt treatment. A veterinarian can confirm the cause and recommend care that has evidence behind it.
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For background on feline health and everyday care, see the other guides on animalso.com.
Keep in mind that this article is general information. It does not replace a consultation about a specific cat.
Note: this overview reflects the 2015 systematic review and the ABCD guideline as published on its site; check the ABCD page for any later revision before relying on it.
Summary of key points for the owner who wants the short version: lysine does not have evidence of benefit for FHV-1, study doses are not dosing advice, and eye or breathing symptoms should go to a veterinarian.
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Frequently asked questions
These answers are included so that you can review the points above in a different form.
Quick Recap
- Does lysine help cats with herpes? The best available review and the ABCD guideline do not find evidence that it prevents or treats FHV-1.
- Is lysine safe for cats? No side effects were reported in the literature the ABCD guideline cites, but that is not proof of safety across products, doses or health conditions.
The Bottom Line
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